Device comprising a television pick-up tube



Aug. 20, 1957 J. c. FRANCKEN DEVICE COMPRISING A TELEVISION PICK-UP TUBE Filed Jan. 29, 1954 N A m 71 m H V U& WL .A MO M m 7 M 3 Y B H 2 3 pany, Inc., New York, N. Y., a corporation of Delaware Application January 29, 1954, Serial No. 407,073 Claims priority, application Netherlands February 2, 1953 7 Claims. (Cl. 250-211) In television picture transmitters a pulse of constant strength is added at regular intervals to the series of pulses constituting the picture or video signal. This pulse serves to maintain constancy of the black level of the picture produced on the screen of the picture tube. If this step is not taken, the mean brightness of the image tends to adjust itself to a constant value irrespective of the nature of the image. Furthermore, unpleasant variations in the brightness level may occur after abrupt variations in the nature of the image.

According to a step which is frequently taken, the said pulse is derived directly from the pick-up tube. The picture signal is obtained by conversion of light into electric current, so that differences in brightness become manifest in currents of different strength, a narrow beam of electrically charged particles moving along parallel lines over the surface intended to assist in the con version. Consequently, if the beam regularly strikes a portion of the surface of which the lighting intensity is invariably the same, the video signal contains a con tinuously recurring fixed value. For this purpose use is made of a strip along the edge of the target, which strip is located at the beginning or the end of the lines and is not struck by light. This step is also taken in pick-up tubes for television, in which the image received by a photo-electric cathode is projected by electronoptical means onto the target, by preventing such a strip from being struck by photoelectrons.

In pick-up tubes of the last-mentioned kind the screening may be effected by providing an opaque strip on the photo-electric cathode, which strip is provided along the edge of the image field in such manner that the electronoptical shadow thereof governs the edge of the target. This step is practicable if the part of the photo-electric cathode that is reproduced is invariably the same.

Pick-up tubes for television are known which corn prise an electron-optical. system of adjustable strength in which such is not the case. Except for the smallest amplification, the photo-cathode is projected only in part on the target with any other adjustment of the optical system. Due to the greater spreading of the electron rays required for the projection, the shadow edge of an opaque strip on the photo-cathode shifts towards the edge of the target with increasing magnification until at last the target is struck by photo-electrons throughout its surface.

The present invention relates to a television device comprising a pick-up tube of the kind which comprises a photo-electric cathode and an electron-optical system of adjustable strength for transmitting the image of the photo-cathode to the target and it has for its purpose to obivate the aforementioned disadvantage. According to the invention, for this purpose the photo-cathode is screened in part from incident light with the use of a movable screen which is impermeable to the image-producing rays and which shifts when the strength of the electron-optical projection lens varies, so that the boundary Patent @fifice 2,803,753 Patented Aug. 20, 1957 between the lighted and non-lighted surfaces is displaced to an extent such that irrespective of magnification a new row strip is invariably located beside the image field on the target at right angles to the scanning lines outside the reach of photo-electrons.

In order to ensure correct positioning of the opaque screen for any adjustment of magnification, it may be connected to the device for adjusting the magnification 'by means of a suitable coupling element. As an alternative, the screen may be displaced under the action of an electro-magnetic device, the energising current in this case being derived from a control magnitude which varies in proportion with the adjust-able magnification.

In certain cases it may be desirable that the surface governed by the photo-electrons should [be smaller than the surface area of the target, so that an edge provided along the periphery of the target is not struck by photo electrons. In this case the screen may have the shape of a diaphragm having a rectangular aperture of variable size, the variation in the size of the aperture being de pendent upon magnification and adjusted by the device for magnification control.

In order that the invention maybe readily carried into effect, it will now be described with reference to the accompanying drawing, showing, by way of example, some embodiments thereof.

Fig. 1 shoWs the use of a mechanical coupling element between the device for magnification control and the movable screen.

Fig. 2 shows the electro-magnetic coupling and Fig. 3 shows the use of a rectangular diaphragm.

A tube comprising a combination of coils constituted by three windings is chosen as an example of a pick-up tube having adjustable magnifification.

I The discharge tube comprises two cylindrical wall parts 1 and 2 of different sections which are connected by means of a conical part 3 of the wall.

The cylindrical part 1 of the smaller section is closed by a flat base 4. The inner side of the base 4 has pro vided on it in known manner, with the interposition of a thin transparent conductive layer, a photo-electric cathode 5 which may consist of caesium-antimony but of which the composition is not described further here.

The wider cylindrical part is closed by means of a base 6 which is slightly spherical for increasing the mechanical strength and which comprises a central support 7 for the target 8 which is arranged in parallel with the photo-electrical cathode 5.

A magnetic system 9 serves for the electron-optical projection of the photo-electric cathode 5 on the target 8. Furthermore, an electron-accelerating field is required which extends from the photo-cathode 5 and which is obtained by providing a difference in potential between the conductive layer under the photo-cathode and a conductive wall 10 which surrounds the electron paths and which terminates at a short distance from the photoeathode. The wall 10 may have the form of a conductive coating on the wall of the tube.

The conical part 3 of the wall has connected to it a cylindrical glass nozzle 11 which communicates with the space enclosed by the parts 1 and 2 of the wall and which is closed at the end remote therefrom. The nozzle 11 contains the electrode system for producing the scanning beam of which the axis is directed towards the target. The electrode system comprises a cathode 12, a control electrode 13, a first anode 14 and an end anode 15, whilst provision is made of deflection means 16 and 17 which enable the electron beam to scan the whole of the surface area of the target. Instead of utilising the electrostatic deflection members 16 and 17, use may alternatively be made of magnetic means.

The magnetic system 9 is constituted by windings 18,

. l9 and 20. Variable resistances 21, 22 and 23 of which the contact arms are arranged on a common shaft 24 and which are operated in common by an adjusting knob 25 serve to adjust the energizing currents in the windings 18 '1 9and Care has been taken to ensure that for any, adjustment-of the knob the energising current provided in. each winding by a current source 26 has the value which is required for the energisation of the windings to produce in common the desired ratio between the image of the photo-cathode and its reproduction on the target.8.

'In order to safeguard a narrow strip 27 along the edge of the target 8 against bombardment by electrons, use is made of. an opaque screen 28 which screens part of the "photo-cathode 5 from incident light of the scene to be taken,"which is projected onto the cathode with the useofi a lens .29, and whichis provided as closely as possibleto the surface 5. The screen 28 is movably arranged and for this purpose comprises a toothed rod 30 which is engaged by a toothed disc 31. The disc 31 may beturned. by the control knob 25. via two discs 32 and33 and'a coupling rope 34, the screen 28 thus being displaced. The part of the. photo-cathode 5 which is screened from incident light must increase in size upon increasing magnification, the boundary of this part shiftingparallel to itself by turningthe control knob 25.

' Another embodiment of the mechanism for moving the screen utilises an electric control magnitude. In the embodiment shown in Fig. 2 it is the current traversing the third coil section20, viz. that which is most adjacent the target, which is suitable for this purpose. The said coil section which counteracts shift of the image as a result of the magnetic projection field for this purpose requires an energising current varying linearly with magnification; This current may traverse directly, or a component thereof, an energising Winding 35 for a pull magnet 36 which displaces the screen 28 against the force of a draw spring 37.

The. use of a diaphragm having a rectangular aperture of variable size may be seen from Fig. 3. The diaphragm comprises two portions 38 and 39 each constituting the half circumference of'the diaphragm. The two parts are movable in the direction of one of the diagonals. This movement may be effected either by means of a mechanical coupling element or electrically by way of a variable electrical value deduced from the projection system. In Fig. 3 theelectriccoupling with the knob for magnification control is established in the manner as shown in Fig. 2. The halves of the diaphragm are coupled by means of a mechanism comprising toothed rods 40 and 41 and an intermediate wheel 42 which transmits the movement of the magnet 36 translating it into a movement of the part 39 which is opposite thereto.

What I claim is:

;1. Television apparatus comprising a pick-up tube, said tube including a photo-electric cathode for producing an electron imagein response to incident light and a target for receiving the electrons from the cathode, electronoptical means for projetcing the image of the photocathode onto said target, means associated with said elec- 2,sos,753 I tron-optical means for controlling and adjusting the magiii nification of the projection of the cathode image, a lightimpermeable movable screen disposed in front of a portion of said cathode to prevent light from impinging on a portion thereof whereby a correspondingly-positioned portion of the target is not struck by electrons frorn'the cathode, and means coupling the movable screen and the magnification control means whereby, regardless of the adjustment of the latter, the movable screen takes up a position at which a portion of the target free from bombardment. by electrons from the cathode always exists.

2. Television apparatus comprising a pick-up tube, said tube including a photo-electric cathode for producing an electron image in response to incident light and a target for receiving the-electrons from the cathode, electron-optical means for projecting the image of the photocathode onto said target, means associated with said electron-optical means for controlling and adjusting the magnification of the projection of the cathode image, means for scanning said target in a given direction with an electron beam, a light-impermeable movable screen disposed in front of a portion of said cathode to prevent light from impinging on a narrow strip thereof whereby a correspondingly-positioned narrow strip of the target at right angles to the-scanning direction is not struck by electrons from the cathode, and means coupling the movable screen and the magnification control means whereby, regardless of the adjustment of the latter, the movable screen takes up a position at which a narrow strip of the target free from bombardment by electrons from. the cathode always exists.

3. Television apparatus as set forth in claim 2 wherein the coupling means between the screen and the magnification control means comprises a mechanical device with a control member actuating the screen and the control means simultaneously.

4. Television apparatus as set forth in claim 2 wherein the coupling means include an electromagnetic device responsive to electrical energy derived from the magnification control means. 1

5. Television apparatus as set forth in claim 2 wherein the screen comprises a fiat, plate-like member extending over one side of the photo-cathode.

6; Television apparatus as set forth in claim 2 wherein the screen comprises a diaphragm having a rectangular aperture and constituted by two members each bounding two sides of the aperture, said members being movable in a diagonal direction. j

7. Television apparatus as set forth in claim 2 wherein the coupling means are adapted to move the screen to cover a wider strip of the cathode when the magnification is increased.

References Cited in the file of this patent UNITED STATES PATENTS 

